Spreader bars are known, in particular in connection with woodworking machines, on which block suction devices are placed which are used for clamping the workpiece to be processed, for example wood panels or the like. Following the placement of the block suction devices on the spreader bars, they are fixed by suction on the spreader bars, so that they are fixed in place. This takes place in that one or several shut-off valves provided in a first vacuum circuit are opened, through which a closed space provided on the underside of the block suction device is connected with a first vacuum circuit. Following the placement of the workpiece on the block suction devices, a vacuum is supplied to a second vacuum circuit and by means of this vacuum the workpiece is held by suction by the block suction device. This occurs in that a clamping space has been provided at the top of the block suction device, which is connected via a connecting line passing through the block suction device with the underside of the block suction device and terminates in the space provided there for the purpose of clamping the block suction device against the spreader bar. This connecting line is connected with the second vacuum circuit. For clamping the workpiece, first the block suction device is positioned and then the workpiece is clamped. These clamping systems have the considerable advantage that the block suction devices can be centrally clamped and the workpieces can be centrally fixed by suction via the block suction devices. There is no need for a hose connection with every individual block suction device, which often has an interfering effect. It is therefore possible to release the workpiece without having to release the block suction device.
However, it has also been found that the spreader bars are relatively expensive, since they need to be provided with two vacuum circuits. In addition, the spreader bars must be provided with respectively two shut-off valves at the locations not occupied by the block suction device, so that the two vacuum circuits can be blocked there. Furthermore, the block suction devices can only be linearly displaced on the spreader bars, i.e. in a direction, which corresponds to the longitudinal extension of the spreader bars. In a direction transversely to this it is necessary to displace the spreader bars in relation to each other. If it is intended to process different workpieces one after the other, this can possibly lead to a considerable adjustment outlay. In addition, as a rule the block suction devices cannot be turned.
Also known are jig tables, on which the block suction devices can be arbitrarily placed. This has the advantage that the position of the individual block suction devices can be exactly matched to the shape of the workpiece. In addition, changes in the position of the block suction devices can be simply performed.
However, it is considered a disadvantage in connection with such jig tables that it is necessary to connect the individual block suction devices via hoses with the vacuum sources. But these hoses have an interfering effect on the processing of the workpiece because, for one, they lie next to the block suction devices on the jig table and are covered by the processing waste. In addition, such hoses can be damaged very easily.